There are a couple stickies at the top of the Bottling/Kegging forum that go into detail about EVERYTHING you need.
Briefly, you need: a CO2 tank, a regulator for the co2 tank(to regulate the co2 pressure, at least one corny keg, one set of quick disconnect fittings for each keg you want on tap at a given time. Some Air hose, some beer hose, some hose clamps, and a party 'cobra' tap(the black plastic tap you see at parties)
That's all that's needed for a simple kegerator. You'd have to open the fridge door to poor a beer. If you want to have a metal tap on the outside, you'll need a shank, and a tap faucet, and then have to drill a hole in your fridge door for the shank.